Drop on demand ink jet technology for producing printed media has been employed in commercial products such as printers, plotters, and facsimile machines. Generally, an ink jet image is formed by selectively ejecting ink drops from a plurality of drop generators or ink jets, which are arranged in a printhead or a printhead assembly, onto an image substrate. For example, the printhead assembly and the image substrate are moved relative to one other and the ink jets are controlled to emit ink drops at appropriate times. The timing of the ink jet activation is performed by a printhead controller, which generates firing signals that activate the ink jets to eject ink. The image substrate may be an intermediate image member, such as a print drum or belt, from which the ink image is later transferred to a print medium, such as paper. The image substrate may also be a moving web of print medium or sheets of a print medium onto which the ink drops are directly ejected. The ink ejected from the ink jets may be liquid ink, such as aqueous, solvent, oil based, UV curable ink or the like, which is stored in containers installed in the printer. Alternatively, the ink may be loaded in a solid form that is delivered to a melting device, which heats the solid ink to its melting temperature to generate liquid ink that is supplied to a print head.
Variations in ink jets may be introduced during print head manufacture and assembly. The variations include differences in physical characteristics, such as ink jet nozzle diameters, channel widths, or lengths, or differences in electrical characteristics, such as thermal or mechanical activation power for the ink jets. These variations may result in different volumes of ink being ejected from the ink jets in response to the same magnitude or same frequency firing signal. To compensate for these differences some previously known printers perform a process to normalize the firing signal for each ink jet within a printhead. Thus, normalizing the electrical firing signals that are used to activate individual ink jets enable all of the ink jets in a printhead to generate ink drops having substantially the same drop mass. In certain instances, an ink jet may fall out of calibration or normalization so that it produces an ink drop that is no longer uniform.
Another issue that arises during operation of an ink jet printer is intermittent, weak, or missing ink jets. Specifically, some ink jets fail either completely or partially so they no longer perform as expected to eject ink onto an image substrate. One method for compensating for such ink jets is disclosed in U.S. Pat. No. 7,021,739 to Burke et al., which is assigned to the assignee of the present application. The method disclosed in that patent disables the inoperative ink jet and uses surrounding ink jets to compensate for the missing, intermittent, or weak ink jet. The printing to be done by the disabled ink jet is performed by one or more of the surrounding ink jets on one or more additional image substrate passes. Thus, this approach slows the printing process because additional substrate passes are required. In another approach described in U.S. Pat. No. 7,448,719 to Roger Newell, which is also assigned to the assignee of the present application, a second or back-up printhead is shifted laterally relative to the substrate so that a properly functioning ink jet is aligned with the defective ink jet along the process direction. In another approach, described in U.S. Pat. No. 7,021,739 assigned to the assignee of the present application, partial nozzle redundancy is used to account for an impaired nozzle. In this approach, the normal ejection output for neighboring nozzles is increased so that the pixels to be printed by the impaired nozzle are printed by neighboring nozzles at previously blank pixels. While these approaches provide temporary relief from a defective ink jet it is still necessary to replace the defective ink jet or printhead.
The replacement of a defective ink jet or printhead can involve significant down time. A new printhead for a solid ink printing machine requires a warm-up time to not only be brought up to the machine operating temperature but to also be sufficiently warmed up to function properly. Failure to be properly warmed up can lead to intermittent missing jets for a printhead. This phenomenon can often be attributed to the process of heating a printhead from cold to the operating temperature, which can cause the ink inside to undergo a phase transition and volume change. This change in the ink can create bubbles and voids in the molten ink within the printhead that can require purging. However, even several purge cycles will not always eliminate air bubbles or voids. Additionally, even after the ink is brought to its operating temperature, bubbles and voids can occur if the ink has not been maintained at this temperature for a sufficient time, thereby resulting in missing jets. Consequently, the current practice is to implement a significant warm-up time for a newly installed replacement printhead, followed by one or more purge cycles before the printing machine is brought on-line. This can result in several minutes of down-time, especially for printing machines have a large number of print heads.